In the chemical industries, chlorine gas (Cl2) is one of the most widely used inorganic chemicals. For example, polyurethanes, halogenated hydrocarbons and white titanium dioxide pigment are commonly manufactured in processes using chlorine gas.
In the latter case of white titanium dioxide pigment manufacture, feedstock is chlorinated with chlorine gas. Chlorinated species are reduced to waste by-products such as: hydrogen chloride (HClgas), hydrochloric acid (HClaq) or inorganic metal chlorides (e.g., FeCl3, FeCl2, MgCl2).
In particular, when titanium tetrachloride (TiCl4) is prepared by the carbo-chlorination of titaniferous ores feedstock (e.g., weathered ilmenite, titanium slag or synthetic rutiles), significant amounts of iron and metal chlorides species are generated as by-products. These by-products may comprise either ferrous or ferric chlorides or a combination thereof, depending on the reaction conditions of the chlorinator. The actual by-products are in fact more complex as these consist of a chlorination waste which is essentially made of a blend of particulate iron chlorides contaminated with unreacted titanium feedstocks, petroleum coke, silica and silicates, and other metal chlorides. The approximate chemical composition of the metal chlorides collected from the cyclones of chlorinators operating with titanium slag only is presented in Table 1 below.
TABLE 1Average composition ranges of the metal chlorides in an as-receivedchlorinator dust, expressed as anhydrous salts (wt. %)Metal chloridesFormulaPercentageIron (II) chlorideFeCl230-90Aluminum (III) chlorideAlCl3 5-15Magnesium (II) chlorideMgCl2 2-20Manganese (II) chlorideMnCl2 1-15Sodium chlorideNaCl1-8Vanadium (IV) oxychlorideVOCl21-6Chromium (III) chlorideCrCl30.5-6 Titanium (III) chlorideTiCl30.1-3 
The formation of these chlorinator wastes has severe economic and environmental implications on the overall process because the wastes must be processed for disposal. Usually, by-product iron chlorides are dumped in large scale deep wells or at sea landfills or simply discharged into wastewater stream. Such discarding involves both environmental issues and a complete loss of the economic value of the chlorine species. Despite being environmentally unsound, these practices are still extensively used at many plant locations, worldwide.
Although attempts have been made to commercialize these by-metallic chloride products as flocculating agent in the treatment of wastewater or as etching agent in pickling baths, these attempts are hampered by the low market value of these by-products. In addition, since the by-products are usually in the form of aqueous solutions, transportation charges are prohibitive.
For these reasons, there has been extensive research on chlorine recycling and various attempts have been made over the past forty years in the titanium dioxide pigment industry to recover the chlorine values from iron chlorides.
In addition, since the introduction in 1998 of the upgrading of titanium slag by high pressure hydrochloric acid leaching, an increasing interest has arose in recovering chlorinated metal values from the spent acid. At present the spent acid is pyro-hydrolysed to regenerate an azeotropic solution of hydrochloric acid leaving behind inert metals oxides that are landfilled as mining residues. The average composition ranges of a spent acid is presented in Table 2 below.
TABLE 2Average composition ranges of spent acidCations orConcentrationchemicals(c/g · dm−3)HCl (free) 40-70Fe(total) 30-60Fe(II) 20-45Mg(II) 10-30Al(III)  4-12Fe(III)  4-12Ca(II)0.5-2V(III)0.5-2Mn(II)0.5-3Cr(III)0.3-2Ti(IV)0.1-1
Until today, there is an absence of a satisfactory industrial process for recovering elemental chlorine from iron chlorides. The main prior art route for recovering chlorine from spent chlorides is the thermo chemical oxidation of iron chlorides in an excess of oxygen.
Thus, several attempts have centered around the oxidation of iron chlorides during which the following chemical reactions are involved:2FeCl2(s)+3/2O2(g)→Fe2O3(s)+2Cl2(g)2FeCl3(s)+3/2O2(g)→Fe2O3(s)+3Cl2(g)
However, until today it has proved very difficult to develop a satisfactory industrial process incorporating the reaction exemplified in the previous equations. Many efforts have been made to overcome the attendant difficulties by conducting the reaction in the gaseous phase such as indicated by Harris et al.1. Harris suggested that ferric chloride can be treated with oxygen in a fluidized-bed reactor in the vapor phase. The process produces chlorine gas, which can be recycled to an ilmenite or rutile chlorination process, and iron oxide by-product rather than soluble chloride wastes.
GB Patent 1,407,0342 discloses oxidation of gaseous ferrous chloride with oxygen in excess at temperatures sufficiently high to avoid condensation of the ferrous chloride.
U.S. Pat. No. 3,865,9203 to RZM Ltd., discloses a process consisting in preheating ferrous chloride at 980° C. to 1110° C. and then oxidizing it by passing pure oxygen to form a mixture of iron chlorides, iron oxide, oxygen and chlorine, which mixture is thereafter cooled and the residual iron chloride converted to iron oxide and chlorine.
The main issues with the full oxidation of either FeCl2 or FeCl3 to iron oxides and chlorine is that thermodynamics requires low temperatures, i.e., usually below 400° C., to shift the equilibrium in favor of the oxidation of the ferric chloride. However it appears that, at low temperatures imposed by thermodynamics, the reaction kinetics becomes too slow whereas at higher temperatures, where the reaction proceeds at a practical rate, the reaction is far from complete.
It was subsequently found that the utilization of a catalyst such as iron oxide accelerates the reaction at lower temperatures. Thus the use of an iron oxide fluidized bed reactor was proposed to lower the reaction temperatures. Actually, U.S. Pat. No. 2,954,2744 to Columbia Southern Chemical Corp. proposed to oxidize ferrous iron chloride by means of air or oxygen at temperatures from 400° C. to 1000° C. in a fluidized bed of iron chloride and optionally iron oxide. Later, in U.S. Pat. No. 3,793,4445 to E.I DuPont de Nemours the oxidation of gaseous iron chloride was performed by passing a mixture of the iron chloride and oxygen through several superposed zones subdivided by walls and in the presence of recycled inert solid particles (e.g., silica sand). During this process, ferrous chloride (FeCl2) is continuously oxidized, first to ferric chloride (FeCl3) and then to ferric oxide (Fe2O3) in one stage. Afterwards, in U.S. Pat. No. 4,144,3166 to E.I DuPont de Nemours, Reeves and Hack improved the process by carrying out the dechlorination reaction in a recirculating-fluidized-bed reactor for example of the type suggested in U.S. Pat. No. 4,282,1857.
However, an additional problem arises during thermal oxidation, that is, the deposition of a solid, dense and hard iron oxide scale (Fe2O3). This scale has a severe tendency to accumulate and adhere strongly on the reactor walls and associated equipment, causing problems in the efficient operation and maintenance of the reactor. Actually, it has been demonstrated that oxide scale occurs above bed level to such an extent that the outlet may become completely clogged in a short time and the operation must be frequently stopped for removing the scale leading to expensive shutdowns. Moreover, serious problems were encountered in increasing the size of the fluid bed reactor towards an industrial scale for this reaction.
Other proposals consisted in operating the oxidation process at lower temperatures using a molten salt bath of NaCl to form a salt complex or eutectic with the iron (NaCl—FeCl3) compound; or conducting the oxidation under a pressure sufficient to effect the liquefaction of the ferric chloride. However, these methods generally require the use of complicated apparatus and the exercise of very careful controls over operating conditions. Furthermore, difficulties seem to be encountered in the removal of by-product iron oxide from the reactor and in the sticking of the particulate bed material.
Another drawback of the thermal oxidation process in general seems to be the poor quality of the gaseous chlorine produced, namely about 75 vol % Cl2 because it is largely contaminated with ferric chloride and other volatile impurities and also strongly diluted with unreacted oxygen (11 vol. % O2) and carbon dioxide (7.5 vol. % CO2). Hence it exhibits a relatively poor commercial value. In addition, immediate recycling to the chlorinator as well as efforts to concentrate the dilute chlorine, involve great additional expenses.
Moreover, efficient chlorine recovery by thermal oxidation requires essentially pure ferrous chloride as feedstock. However, the mechanical separation of the particulate ferrous chloride from the major contaminants (i.e., coke) in chlorinator dust is a hard task. In fact, if thermal oxidation of impure ferrous chloride is carried out at temperatures in excess of 800° C., the coke present in the dust is burned up, thereby producing hot spots in the reactor, which leads to the sintering of the iron oxide accompanied by a build-up of the oxide on the walls, which in turn leads to clogging within a short time.
After the unsuccessful pilot and pre-commercial trials made by E.I. Du Pont de Nemours for thermal oxidation, other titanium dioxide pigment producers investigated this technology such as SCM Chemicals Ltd.8, Kronos Titan GmbH9 and recently Tioxide10.
Another route, namely the electrolytic route, was considered for recovery of both chlorine and iron values.
It appears from the prior art that work has been done on the electrodeposition of iron metal from iron-containing solutions since the second half of the eighteenth century. In fact, various processes for electrowinning, electroplating, or electrorefining iron metal are known. Usually, the aim of these processes is to prepare an electrolytic iron with a high purity and to a lesser extent pure iron powders. Usually, the most common electrolytes were based on iron sulphate and to a lesser extent with iron chlorides.
Most of the known electrochemical processes were originally designed to electrodeposit iron at the cathode while the anodic reaction usually consisted in the anodic dissolution of a soluble anode made of impure iron. In such processes, the use of consumable-type anodes seems to have generally allowed avoiding an undesirable evolution of corrosive nascent oxygen or hazardous chlorine gas.
On the anode side, chlorine recovery by electrolysis from brines or by-produced hydrochloric acid is well-documented technology with many plants operating worldwide with a discrete number of electrolytic processes. However an industrial scale electrochemical process that combines the two principles of recovering directly both iron and chlorine from waste iron-containing chlorides does not seem to exist.
The first well-documented attempt apparently dates back to 1928 with the patents of LEVY11. The inventor disclosed a simple electrochemical process for recovering both nascent chlorine and pure electrolytic iron from a solution of pure ferrous chloride. The electrolyser was divided with a diaphragm as separator made of porous unglazed clay to prevent the mixing of products. The electrolysis was conducted at 90-100° C. under a current density of 110-270 A·m−2 with an average cell voltage of 2.3-3.0 V. The Faradaic current efficiency was 90-100%. The anolyte was a concentrated chloride solution (e.g., CaCl2, NaCl) while the catholyte was an aqueous solution containing 20 wt. % FeCl2. The anode was carbon-based while the cathode was a thin plate, mandrel or other suitable object.
More recently, in 1990, OGASAWARA et al. from Osaka Titanium Co. Ltd (now Toho)12 disclosed in a patent application an electrolytic process to produce iron and chlorine through the electrolysis of an iron chloride-containing aqueous solution (an effluent resulting from the pickling of steel or from the process of producing titanium tetrachloride or nonferrous titanium ore) by the use of anion and cation exchange membranes in conjunction with a three-compartment electrolyser. In this process as exemplified in Ogasawara, the catholyte, which is made of high purity ferrous chloride and constantly adjusted to a pH of 3 to 5 with ammonia, and the anolyte made of sodium chloride, recirculate in loop inside their respective compartments, while the iron-rich chloride-containing solution to be electrolysed circulates through the central compartment, that is, the gap existing between the two ion-exchange membranes. The cathode used is preferably iron but may also be stainless steel, titanium or titanium alloy, and the anode used is made of insoluble graphite. According to the inventors, this 3-compartment process apparently allows, in contrast to that using a two-compartment electrolytic process, to avoid polluting the resulting electro-crystallized iron by embedded impurities such as metal oxides. In addition, maintaining the catholyte pH between 3 and 5 allows avoiding hydrogen evolution at the cathode.
However, in such process, there appears a high ohmic drop due to (i) the additive resistivities of the ion exchange membranes and (ii) the associated gap existing between the two separators. In addition, the utilization of a graphite anode combined with a sodium chloride brine anolyte seems to cause a high overpotential for the reaction of chlorine evolution. Both the high ohmic drop and the anodic overvoltage contribute to the cell potential. This therefore leads to a high specific energy consumption for both chlorine and iron recovery, which is not compatible with a viable commercial process.
Therefore remains a need for an efficient and economical process to recover both iron metal and chlorine gas from iron-rich metal chloride wastes.
The present description refers to a number of documents, the content of which is herein incorporated by reference in their entirety to the extent that the incorporated subject-matter is not contradictory with the explicit disclosure herein.